The present invention relates to an optical pick-up apparatus to record/reproduce the optical information by converging the light flux emitted from a laser light source by an objective lens onto the information recording surface through a transparent substrate, and to a correction lens in a converging optical system. Particularly, the present invention relates to an optical pick-up apparatus structured such that recording/reproducing is conducted by one objective lens for the first optical information recording medium in which the thickness of the transparent substrate is t1, and for the second optical information recording medium in which the thickness of the transparent substrate is t2 (t2>t1), and at the time of recording/reproducing of the second optical information recording medium, the diverging light flux is entered into in the objective lens, and to a correction lens.
Recently, as a short wavelength red semiconductor laser is put to practical use, a DVD (digital versatile disk) having the large capacity and high density, whose dimensions are almost the same as the conventional CD (compact disk), comes into the market. In this DVD, as the numerical aperture (NA) of the objective lens when the short wavelength red semiconductor laser of 635 nm or 650 nm is used as the light source, about 0.6 is necessary.
Further, as a CD-R which is a recordable compact disk, spreads, the interchangeability with the CD-R is also required, as the optical pick-up apparatus. Because the reflection rate of the CD-R is reduced on the short wavelength side of 635 nm or 650 nm, and the necessary signals (a reproduction signal, focus error signal, and tracking error signal) can not be obtained, the semiconductor laser of 780 nm is prepared for these CD-Rs, separately from the short wavelength semiconductor laser for the DVD.
For the optical pick-up apparatus to record/reproduce these plurality of optical disks, the light sources for each of optical disks are prepared, however, for the sake of simplification of the optical pick-up apparatus, it is required that the interchangeability of one objective lens for each optical disk is secured. In order to secure the interchangeability, irrespective of the wavelength of light source of each optical disk, and the thickness of the transparent substrate of each optical disk, it is necessary that, at least, a good converging spot is formed on the information recording surface.
However, in the case where the thickness of the transparent substrate is different, when the magnification m of the objective lens is the same in each optical disk, at least in one optical disk, the spherical aberration is generated. The dotted line in FIG. 7 shows the spherical aberration of the CD when, for both of the DVD and CD, m=0, and the objective lens is designed such that the spherical aberration is not larger than the limit of the diffraction (0.07 λrms) at the time of the DVD recording and reproducing. When such the spherical aberration is generated, the conversing spot on the information recording surface is deteriorated.
Therefore; for correction of the spherical aberration generated when the thickness of the transparent substrate is different, the technology by which the magnification of the objective lens is changed when the DVD is used, different from the case of the use of the CD, is used. The solid line in FIG. 7 shows the spherical aberration when the magnification of the lens at the time of the use of the CD is m=−1/17.5 (the incidence of the diverging light to the objective lens). Herein, the numerical aperture NA 1=0.60 in the actual use of the DVD, and the numerical aperture NA2 =0.45 in the actual use of the CD are shown on the vertical axis in FIG. 7.
However, in the optical system in which the light flux from the definite distance enters the objective lens, the following disadvantages occur. That is, because the diverging spherical wave enters the objective lens, when the objective lens shifting occurs in the tracking, because the light flux obliquely enters the objective lens due to the shifting of the optical axis, the coma is generated.
FIG. 8 is a view showing the coma generated by the objective lens shifting in the above magnification m=−1/17.5 and the focal distance f=3.5 mm. That is, when the diverging light enters the objective lens, the spherical aberration can be corrected, however, in this case, the coma generated by the objective lens shifting becomes a new problem. Also by the generation of this coma, the optical spot formed on the information recording surface of the optical disk is deteriorated, and which is not preferable in the recording/reproducing of the optical disk.